1. Field of the Invention
The present invention relates to an attachment structure for the main spindle and the tool holder of a machine tool.
2. Description of the Related Art
A machine tool typically has a tool clamping mechanism for securing a tool holder to the main spindle in a removable manner. The tool clamping mechanism generates force for gripping the tool holder using a spring or the like. FIG. 3 generally illustrates a conventional tool clamping mechanism, which includes a main spindle 1, a drawbar shaft 3, a tool holder 7, and a spring 16. The drawbar shaft 3 is inserted in the main spindle 1 with the spring 16 interposed between them.
A machine tool also includes a structure to supply high-pressure cutting fluid from the tip of the tool to a machining point, which is where the tool attached to the tool holder contacts the workpiece, in order to improve machining performance. Such a structure includes a pump for feeding high-pressure cutting fluid in order to supply high-pressure cutting fluid to the machining point from the tip of the tool, with a through hole formed in the main spindle and/or the tool holder through which cutting fluid flows.
Action of the high-pressure cutting fluid on a tapered portion and/or a pull stud of the tool holder generates force in a direction that decreases the force with which the tool clamping mechanism grips the tool holder. If the pressure of the cutting fluid becomes high, cutting fluid penetrates between the tapered hole of the main spindle and the tapered portion of the tool holder, causing a change in the distance between the tapered hole of the main spindle and the tapered portion of the tool holder. Such a change in the distance between the tapered hole of the main spindle and the tapered portion of the tool holder can affect the machining accuracy.
As solutions to this problem, some structures have been proposed. For instance, Japanese Utility Model Application Laid-Open No. 1-92345 discloses two types of structure as attachment structures for the main spindle and a tool attachment portion of a machine tool. One of the attachment structures has a sealing mechanism formed of a sealing element in the tapered portion of the tool holder. The other attachment structure realizes a sealing mechanism by providing an annular component having a groove formed in the circumferential direction in the inner surface of the main spindle's accommodating hole and providing a sealing element on the annular component.
The former attachment structure disclosed by Japanese Utility Model Application Laid-Open No. 1-92345, i.e., with a sealing mechanism formed of a sealing element in the tapered portion of the tool holder, is complicated and expensive because a sealing element need to be provided in the tapered portion of each tool holder.
The latter structure disclosed by Japanese Utility Model Application Laid-Open No. 1-92345, i.e., the one realizing a sealing mechanism by providing an annular component having a groove formed in the circumferential direction in the inner surface of the main spindle's accommodating hole and providing a sealing element on the annular component, is also complicated and expensive due to the necessity to separately prepare the annular component.